ADDRESSING FUTURE CHALLENGES TO REDUCE PFCS EMISSIONS FROM ALUMINUM SMELTERS

Transcription

1 1 ADDRESSING FUTURE CHALLENGES TO REDUCE PFCS EMISSIONS FROM ALUMINUM SMELTERS International Conference on Natural Hazards and Disaster Management June 2017, Osaka, Japan

2 - If the Earth is in a thermal steady state: T= 256 K - Real average: 288 K. - This difference is due to natural greenhouse effect - Natural greenhouse gases: H 2 O, CO 2, N 2 O, CH 4 - Human Enhanced Greenhouse effect: Excessive amounts of natural GHG gases and xenobiotic GHG gases 2

3 Primary sources of greenhouse gas emissions 3 Electricity production (31% of 2013 greenhouse gas emissions) - Electricity production generates the largest share of GHG emissions. Transportation (27% of 2013 greenhouse gas emissions) - GHG emissions from transportation primarily come from burning fossil fuel. Industry (21% of 2013 greenhouse gas emissions) GHG emissions from industry primarily come from burning fossil fuels for energy from certain chemical reactions necessary to produce goods from raw materials. Commercial and Residential (12% of 2013 greenhouse gas emissions) GHG emissions arise primarily from fossil fuels burned for heat, the use of certain products that contain greenhouse gases. Agriculture (9% of 2013 greenhouse gas emissions) GHG emissions from agriculture come from livestock and rice production.

4 4 Aluminum production cycle 2 main inputs in primary aluminum : alumina and energy 2 main features of the production process: Energy use Environmental impact

5 Aluminum production cycle 5

6 6 Air emissions From Aluminium Industry Fluorides (HF & particulate) are mainly emitted from the pots during interventions in the pots (like anode changing). Sulfur dioxide (SO 2 ) is emitted mainly from oxidation of the sulfur content in the anodes. Dust is mainly emitted from pot lines and material handling systems. Anode production is a source of polyaromatic hydrocarbons (PAH), but more efficient systems have reduced these emissions to a minimum.

7 Fluoride emissions from aluminium smelters 7

8 8 Fluoride emissions from aluminium smelters Generation of unwanted fluoride by-products from the aluminium smelting process is an unfortunate reality of the current state technology. The release of these fluorides into the potrooms and surrounding environment is unacceptable and must be minimized. There are legal, health, environmental and operational performance issues which drive all smelters to consider effectively.

9 9 GHGs Emissions from Aluminium production The major environmental impact of refining and smelting is greenhouse gas (GHG) emissions. Greenhouse gas emissions in the production of primary aluminium come from processes such as coking, anode production and consumption, and electrical generation. The greenhouse gases resulting from aluminum production include, among others, carbon dioxide (CO 2 ), and perfluorocarbons (PFCs).

10 10 GHGs emissions from Aluminium industry Secondary smelting Recycle/ Remelt Fabrication Primary - Direct (PFC, Carbon, Fuel for Alumina, etc.) Primary Direct (PFC, Carbon, melting, etc.) Primary - Indirect Electricity Supply Primary Indirect (Electricity consumption) Primary smelting is the most polluting stage of aluminum production

11 Primary Aluminium production and PFCs emissions Primary aluminium is produced using the Hall-Héroult electrolytic process. In this process, the smelting pot itself acts as the electrolysis cell during the reduction process. The pot itself forms the cathode, while the anode consists of one or more carbon blocks suspended in it. Molten aluminium is evolved while the anode is consumed in the reaction as follows: 2 Al 2 O 3 (sl) + 3 C(s) 4Al(l) + 3 CO 2 (g) 11

12 GHGs From Primary Aluminium Production 12 CO 2 Alumina mining & refining 2 t CO CO 2 eq/t 2 Al Electricity Input 15.6 MWh/t Al 4.8 t CO 2 eq/t Al CO 2 Anode fabrication 0.3 t CO 2 eq/t Al Gases Feeder Anode Electrolyte Molten Aluminium Cathode Block CF 4 and C 2 F 6 PFCs Emissions 1.2 t CO 2 eq/t Al CO 2 Anode Carbon 1.7 t CO 2 eq/t Al PFCs are potent GHGs, 5,000 to 10,000 times more powerful than CO 2 Global average about 11 t CO 2 eq/t Al

13 13 Primary Aluminium production and PFCs emissions The aluminium production process is the largest anthropogenic source of emissions of two PFCs: CF 4 and C 2 F 6. PFCs emissions are expected to increase at a slower rate than aluminium production because PFCs emission factors are expected to decrease over time. Reductions in PFCs emission factors are anticipated by using modernized smelter technologies Large efforts to reduce PFCs emissions from aluminium smelting.

14 Primary Aluminium production and PFCs emissions 14 GWP: Global Warming Potential

15 Primary Aluminium production and PFCs emissions When the alumina ore content of the electrolytic bath falls below critical levels required for electrolysis, rapid voltage increases occur, termed anode effects. Anode effects cause carbon from the anode and fluorine from the dissociated molten cryolite bath to combine, producing CF 4 and C 2 F 6 due to the following reactions: Na 3 AlF 6 + ¾ C Al + 3NaF + ¾ CF 4 Na 3 AlF 6 + C Al + 3NaF + ½ C 2 F 6 The primary PFC emissions pathway is the exhaust duct collection system, which removes gases from the pots. 15

16 Primary Aluminium production and PFCs emissions The frequency and duration of anode effects depend primarily on the pot technology and operating procedures. Emissions of CF 4 and C 2 F 6, therefore, vary significantly from one aluminium smelter to another depending on several factors. The factors that can potentially influence the PFC generation rate are: Cell Technology Type Feed Delivery System Cell Operating Parameters Anode Effect Kill Routine Electrolyte Properties Cell Liquid Velocities Alumina Quality Anode Coke Smelter Configuration 16

17 Primary Aluminium production and PFCs emissions The methods, assumptions, and data used to estimate PFCs emissions from primary aluminium production are not clear, not transparent or not well documented. Lack of transparency in reporting makes impossible to review, compare, or verify emission estimates. It is possible to develop a smelter-specific relationship between emissions and potentially relevant operating parameters. PFC emissions from primary aluminium production should be calculated on a smelter basis. 17

18 Primary Aluminium production and PFCs emissions Perfluorocarbons (PFCs) are greenhouse gases with atmospheric lifetimes of more than 1000 years. PFCs have greenhouse effects 6,500 to 9,200 than CO 2. Even there mixing ratios are very small (ppb and ppt levels in air), they are highly effective greenhouse gases. They are powerful greenhouse gases and today's emissions will still be affecting earth's climate in the next millennium. The only known sinks for these greenhouse gases are light destruction (photolysis) or ion reactions in our mesosphere. A new and worrying development has been the discovery of a hybrid greenhouse gas derived from PFCs and SF 6 (SF 5 CF 3 ). SF 5 CF 3 is the most powerful greenhouse gas discovered and whose concentration is rising rapidly. 18

19 19 Monitoring and control of PFCs emission The good practice method requires developing or choosing a smelter-specific relationship between PFCs emissions and relevant operating parameters. In order to develop the relationship between emissions and process parameters, operational data must be compiled simultaneously with the emissions measurements. To demonstrate the applicability of an estimation model, emissions must be measured in the smelters at least once to ensure the applicability of the model.

20 20 Monitoring and control of PFCs emission The choice of sampling method, sampling scale, and analytical methods is critical in developing a robust relationship. Analytical system must be appropriate for the sampling method, stable, and free of (or corrected for) interferences. The detection limits of the analytical system must be sufficient to detect PFCs concentrations in the exhaust duct and potroom roofs. Gas chromatography is a selective method for qualifying and quantifying PFCs.

21 Monitoring and control of PFCs emission The lowest detection limit for PFCs are obtained with FTIR analysis performed within particular conditions. Detection limits of 0.7 ppbv for CF 4 and 1.1 ppbv for C 2 F 6 were measured with a 10-meter path length gas cell. The sampling can be done by using gas bag or canister. The lifetime of a sample is about 48 hours for a gas bag and 30 days for a canister. Use of thermal desorption of PFCs is a novel approach for the characterization of fugitive and duct exhaust emissions from primary aluminum smelters. 21

22 22 Passive Absorber For sampling The detection limit and uncertainty: LOD is 7 g/m 3 for 24 hours exposure The uncertainty at 2 is 4.5 % over the whole exposure range.

23 23 Online monitoring Continuous measurements using Lasers Lasers for HF and Dust already commercially available Continues measure of air flow, either through ultrasonic or by laser PFC laser system coming up and will become a good alternative

24 Conclusions Today s sampling standards are being challenged by both need for accuracy (reduced levels, detection limits) and the future need for same time information. The future emission control will trend towards online monitoring. Long term sampling using passive absorbers can be utilized as manual verification of online systems The demand for PFC control will demand development both on sampling techniques and online systems, absorbers would ease the sampling Environmental data s must become a part of the operational parameters, and not only a reporting value Online monitoring includes emission values into process variables and can both save environment but also preserve value 24

25 25 Acknowledgement Prof Mariem Al-Maadeed, CAM-VP R&D Qatar University Prof Geir Martin Haarberg, NTNU-Norway Dr Are Dyrøy, Environmental Scientist, Hydro-Norway Dr Chris Nevadas, Hydro-QSTP Mr Jasim Al Mejali, Qatalum

26 Thank you for your attention 26